The present invention relates to improvements in forming of particulates of metals, metalloids and alloys.
For many applications it is necessary that metals, including metallic alloys, and metalloids such as silicon and its alloys be provided in particulate form. Many systems have been devised for doing this. Among these are centrifugal atomizers which exist in various forms. In one form of centrifugal atomizers the material to be atomized is fed onto the surface of a rotating disc-like member which may be dished or flat. In one form of such systems, a gas is used to cool the particles thrown off the rotating member by centrifugal forces. Representative of this type of system are U.S. Pat. Nos. 2,752,196, 4,053,264 and 4,078,873. Other systems rely on contact of molten droplets with a cooled surface.
The prior art systems known to applicants suffer from several disadvantages, especially when the metals or metalloids being processed have a high melting point. One disadvantage when gases are used for cooling is the volume of gas which must pass through the system to provide sufficient cooling capacity for solidification of the particles. Another disadvantage lies in the need for materials of construction of the apparatus which will withstand the temperatures encountered.
Additionally it has been discovered that properties of some alloys are altered by the speed with which the materials are cooled from the molten state. It is known that rapid cooling can be used to make amorphous alloys or metallic glasses. Some of the metallic glasses have been shown to exhibit properties which are quite different from the same materials in the crystalline state. A discussion of these materials is given in an article entitled "Metallic Glasses" by John J. Gilman, appearing in Science, volume 208, 23 May 1980 at pages 856-861, and in an article of the same title by P. Chaudhari, B. C. Giessen and D. Turnbull appearing in Scientific American, Volume 242, (No. 4), April 1980 at pages 98-118.
In U.S. Pat. No. 4,377,199 granted Aug. 31, 1982 to Speier (one of the named inventors herein) and Gentle there is described a method and apparatus which provide a centrifugal atomizer making use of the heat of vaporization of liquid coolant and which thereby provides a system which offers rapid cooling with the temperature of most components under equilibrium conditions at or near boiling point of the coolant liquid used.
More specifically, the invention described in the patent comprises rotating a horizontally mounted disc-like member at high speed, introducing a stream of volatile liquid coolant at the center to provide an outwardly flowing film of coolant over substantially the entire upper surface of the rotating member and introducing the material to be atomized into the coolant film at a point spaced from the center. The molten material and the rotating member are cooled by evaporation of coolant, and particles are thrown from the device by centrifugal force. A modification of the rotating member described in the patent provides upwardly projecting vanes around the periphery of the rotating member which collide with the particles causing them to be flattened and resulting in a high surface area particulate.
While the above-described patented invention serves to produce a desirable particulate material for some applications a relatively broad range of particulate size is produced. It is desirable for some applications to produce a larger percentage of relatively finer particulate product, i.e., less than 325 U.S. Standard Mesh screen size. Additionally it was found that there was some degree of splashing of the metal or metalloid upwardly from the rotating disc-like member onto the housing of the unit, necessitating periodic cleaning of the housing.